A sensor wire assembly is known from EP-1,774,905 assigned to the assignee of the present application for measuring a physiological variable, in particular pressure, inside a patient's body. The pressure sensor used in the EP-document is a miniaturized semiconductor sensor and one purpose of the assembly of EP-1,774,905 is to handle the electrical configurations of these miniaturized semiconductor sensors that are not always compatible with the transducer amplifiers in existing patient monitors. For instance, the miniaturized sensors often cannot operate over the entire range of excitation signal magnitudes and frequencies found among the various types of patient monitors. Thus, they cannot be connected directly to many of the patient monitors already in use. To be used with such existing monitors, a specialized interface must be placed between the sensor and the monitor. Such an arrangement necessitates additional circuitry on the interface and, because existing monitors have been designed to provide only limited amounts of power, the additional circuitry may require an independent source of electrical power. As a consequence, use of the newer miniaturized sensors often adds cost and complexity to the overall system.
In addition, because of the above limitations, these sensors must often be configured to generate an output signal which is proportional to the pressure sensed, but that is not related to the excitation signal, supplied to the sensor by the monitor, in a way that is directly usable by the physiology monitor, e.g. the sensitivity may be different. As discussed, this does not conform with the electrical format required by the many monitors that are commercially available and already in widespread use. As such, the newer sensors can only be used with specific signal conditioning and displaying units, thereby requiring additional equipment to be purchased. This is especially undesirable given the cost sensitivities so prevalent in today's health care environment.
U.S. Pat. No. 5,568,815 discloses an interface circuit for interfacing a sensor to a patient monitor. The interface circuit includes a power supply circuit that receives an excitation power signal generated by the patient monitor, and derives therefrom unregulated and regulated supply voltages for use by the electrical components on the interface circuit. Further, the power supply circuit generates an appropriate sensor excitation signal. The interface circuit further includes receiving circuitry for receiving a sensor output signal generated by the sensor. A scaling circuit then scales that signal into a parameter signal that is proportional to the physiological condition detected by the sensor, and that is also proportional to the excitation power signal generated by the patient monitor. An obvious drawback of the device of U.S. Pat. No. 5,568,815 is that, in order to connect the sensor to the monitor, a separate additional unit in the form of the interface circuit is required.
A similar solution is disclosed in U.S. Pat. No. 6,585,660 that relates to a signal conditioning device that interfaces a variety of sensor devices, such as guide wire-mounted pressure sensors, to physiology monitors. The signal conditioning device includes a processor for controlling a sensor excitation and signal conditioning circuitry within the signal conditioning device. The processor also supplies signals to an output stage on the signal conditioning device representative of processed sensor signals received by a sensor interface of the signal conditioning device. Power for the signal conditioning device processor is supplied by an excitation signal received from a physiology monitor that drives the output stage. In addition, a temperature compensating current source provides an adjustment current to at least one of a pair of resistive sensor elements to compensate for differences between temperature change upon the pair of resistive sensor elements, thereby facilitating nullifying temperature effects upon the resistive sensor elements.
The Association for the Advancement of Medical Instrumentation (“AAMI”) has defined power requirements for physiology monitors and in particular the input/output connector to a sensor wire assembly must comply with the standard set by American National Standards Institute (“ANSI”)/AAMI BP22-1994 (referred to as “BP22” in the following). According to the BP22-standard, an input/output connector arranged at the proximal end of a five line connector cable includes a pair of differential output signal lines. The output signal lines are driven by a sensor adapting circuitry's output digital to analog converters (discussed further herein below). The differential output signal, by way of example, operates at 5 μV/mmHg/VEXC. An operation range of −150 μV/V to 1650 μV/V therefore represents a sensed pressure range of −30 to 330 mmHg. An exemplary resolution (minimum step) for the differential output signal is 0.2 mmHg.
A pressure measurement procedure when a pressure wire provided with a pressure sensor at its distal end is inserted into a patient's blood vessel, e.g. into the cardiac veins of the heart to measure local constrictions may last for several hours. The inventor has noted that variations and changes of the environmental air pressure in the operation room may have considerable impact on calculations based upon the measurement results received from the pressure sensor at the pressure wire inside the body in relation to environmental air pressure, which in particular may be significant if measurements are to be performed for longer time periods, e.g. several hours.
In order to fully appreciate the present invention a short background of the entire measurement procedure will be presented.
One important application of a pressure sensor wire is to be able to identify constrictions of coronary vessels, e.g. in the great cardiac vein, by performing so-called Fractional Flow Reserve-measurements (FFR). In short FFR is determined by obtaining the ratio between the pressure distally and proximally of a constriction. In practice a catheter is initially inserted and positioned with its distal part e.g. in the aorta. The distal part of the catheter is provided with a pressure sensor of a conventional type having a sensor typically positioned external to the patient's body and placed in fluid communication with the body cavity where the distal part of the catheter is positioned via a fluid-filled catheter line. Pressure variations within the body cavity are then indirectly communicated to the diaphragm at the pressure sensor by way of fluid contained in a lumen running within the entire catheter length and having a distal opening close to the distal end of the catheter. As such, the accuracy of such systems has suffered due to variations in hydrostatic pressure and other inconsistencies associated with the fluid column.
When the catheter is correctly positioned, the pressure sensor wire is inserted into another lumen of the catheter and advanced until the pressure sensor at the distal end of the pressure sensor wire is in, or close to, the distal opening of the lumen. The calibration of the pressure sensor of the pressure sensor wire in relation to the pressure measured by the catheter pressure sensor is then performed. When the calibration has been performed the pressure sensor wire is further advanced until its pressure sensor element is positioned distally of the constriction, and the assembly is then ready to measure the pressures and determine FFR.
Since the pressure sensor element is calibrated to the ambient pressure at the start of the measurement procedure, changes of the ambient pressure during the procedure may have significant effect of the accuracy of the calculated FFR as the measured catheter pressure reflects these changes whereas the pressure from the calibrated pressure sensor element does not. Thus, the catheter pressure sensor measures pressure in relation to the ambient pressure, whereas the pressure sensor of the pressure sensor wire measures the absolute pressure. During the calibration process the pressure sensor wire pressure is set equal to the catheter pressure sensor pressure.
Thus, the object of the present invention is to achieve an improved pressure sensor assembly that generates reliable pressure measurement values irrespectively of external pressure variations, and that is easy to use and has a low over all cost.